Desiccant wheels and energy recovery wheels are two types of wheels used in HVAC, or for conditioning process air. Desiccant wheels are used to transfer moisture from one air stream to another. Desiccant wheels are of two types: “active” and “passive”.
“Active” desiccant wheels use an external heat source to heat one of the air streams, to reactivate/regenerate a portion of the wheel. “Active” desiccant wheels have been generally used for industrial applications requiring high moisture removal, but are being increasingly used in commercial HVAC applications. Examples of active desiccant wheels and systems are disclosed in several patents e.g. U.S. Pat. Nos. 6,311,511; 5,551,245; and 5,816,065.
“Passive” desiccant wheels do not use an external heat source and rely on the relative humidity difference between two or more airstreams to drive moisture transfer between the air streams. Examples of “passive” desiccant wheel systems and use are disclosed in U.S. Pat. Nos. 6,237,354 and 6,199,388.
As thermally activated desiccant wheel systems use substantial energy (steam, electric, gas etc.) to reactivate or regenerate the wheel, various methods have been adopted in the past to minimize the use of reactivation energy with various control methods and/or use of additional components. Methods such as using heat recovery devices to transfer heat energy from process air to reactivation inlet air, or to transfer heat energy from the outlet of reactivation air to the inlet of reactivation air, have resulted in excessive add-on costs.
Dehumidification is a process of removing moisture from air. There are several known methods of dehumidifying air. The two commonly used involve refrigeration, desiccants, or both. In the case of dehumidification using refrigeration, moisture from an airstream passed over a cooling coil condenses, thereby reducing the moisture in the air stream. In the case of dehumidification using desiccants, the process is one of absorption or adsorption. For absorption, either liquid or solid desiccants are used, typically halide salts or solutions. For adsorption, solid desiccants like silica gel, activated alumna, molecular sieve, etc. are used.
Desiccant based dehumidifier systems can be either the multiple tower, cyclic type, or the continuously rotating type. The air to be dried is generally referred to as process air and the air used to regenerate the desiccant is referred to as regeneration or reactivation air. The terms regeneration and reactivation will be used interchangeably in this specification.
In practice, refrigeration based dehumidification systems are limited in the moisture they can remove, because attempting to reach a dew point humidity below freezing often leads to frost buildup on the cooling coil. Avoiding this, or coping with it, leads to making the system more complex and often necessitates reheating.
Desiccant dehumidifier systems, on the other hand, work independently of the dew point, and hence can achieve very low dew point humidities, necessary for many industrial applications. Some examples of their use are in pharmaceutical production areas and food processing areas. They are well suited for uses which require relative humidities or dew point humidities lower than those that can be technically and economically achieved through refrigeration alone.
Further, hybrid systems using both refrigeration and desiccant units are commonly used and help reduce energy usage and provide simple and reliable operation of the whole dehumidification system.
Compared to refrigeration type dehumidification units, desiccant dehumidifiers usually use more heat energy, mainly for regeneration. Accordingly, many desiccant equipment configurations and control strategies have developed for capacity reasons, and energy control to minimize energy use.
Generally, desiccant dehumidifier units, for air at atmospheric pressure, are of the rotary type. The desiccant is contained in a rotary bed, also referred to as a wheel. The term wheel will be used in this specification, to be understood to be a rotary bed having desiccant. Sometimes the term rotary bed, or just bed alone, may be used. The wheel moves on a continuous or intermittent basis, through, typically, two compartments (often referred to in the industry as sectors), one for process, and the other for regeneration. In the process sector, the process air passes through the wheel and is dried by contact with the desiccant. In the regeneration sector, air is brought in, generally from atmosphere, passed over a heat source to elevate (raise) its temperature, then passed through the remaining portion of the wheel that is referred to as the reactivation or regeneration sector. This heats the wheel and drives out the water. Typically the process sector is 50 to 80% of the total bed/wheel area, though it could be more or less, the remainder being the reactivation sector.
Often, another sector is added between the process and regeneration sector, and is referred to as the purge sector. A third airstream (generally called the purge air) is passed through the purge sector and becomes a portion of the regeneration air. The incorporation of the purge sector helps to recover some residual heat from the rotating wheel before it enters the process sector, thereby reducing the overall energy requirement for regeneration, as well as improving the overall moisture removed by the wheel.
When working in the field of desiccant wheels and driers or systems that use them, it is customary to describe systems in terms of flowcharts or schematics that may lead a layperson to believe the wheel is physically split into separate sectors. However that is not the case. The desiccant wheel is uniform and without separators. Each square inch on the surface of the wheel is substantially the same as another square inch. So a portion of the wheel referred to as, for example, the regeneration sector (sometimes referred to as a portion, or zone) is actually the area of the wheel that happens to be passing through that air stream sector (sometimes referred to as a portion, or zone) at the moment. Thus, for example a particular point of the wheel may pass through the process sector, a purge sector, and a regeneration sector encountering different air conditions as it does so. Those air conditions may be taking moisture from, or adding moisture to, the desiccant material.
In typical desiccant dehumidifier units, the process air flow rate and the reactivation flow rate are generally fixed and are set or adjusted with the help of manual or automatic dampers. This can be improved upon.
In the design of a typical dehumidifier system for controlling the humidity in a given space, the airflow needed to control the space temperature may often be more than the quantity of dehumidified air needed to control the space humidity. In such cases, a portion of the process air is typically bypassed around the dehumidifier unit, and is then combined with air exiting the dehumidifier unit. Then the combined air is cooled (or heated), and supplied to the controlled space.
As desiccant dehumidifier systems inherently use a significant amount of heat energy for regeneration, efforts have been made to find ways to reduce the amount of heat used by the system.
One well-known system and method used is to control the heated temperature of the regeneration air before it enters the reactivation sector of the wheel.
Another well-known method is to control the regeneration heat input amount by controlling the air temperature leaving the reactivation sector.
Depending upon the type and amount of relative humidity and dew point control, when the space or air condition is satisfied, the control strategy may employ the start/stop of the dehumidifier. Similarly, use may be made of automatic dampers to continuously vary the amount of air bypassing the dehumidifier unit to satisfy the operation and design needs.
The correlation of the process and reactivation sector area, the wheel rotating speed, the relative process and reactivation air flow rates and velocities through the two sectors, have in the recent decade been documented in Japan, India and USA resulting in robust mathematical modeling tools regularly used for the design, selection, and incorporation of a desiccant wheel, in a finite way, in a dehumidifier unit. Such tools are being used regularly to optimize a dehumidifying system at the design and build stage.
One such study and development of a mathematical model is detailed in a document “Modeling of Rotary Desiccant Wheels” by Harshe, Utikar, Ranade and Pahwa, in 2005.
In the case of rotating desiccant dehumidifier units, it has been known that equipment performance at the design and build stage can be optimized by using such a mathematical modeling tool, to select a particular percentage as reactivation sector, as well as the process and reactivation flow rates, and also a given bed rotational speed. In such cases, under part loads and instantaneously changing moisture load, dehumidifier capacity control is achieved by using the traditional control strategies described above, some of which are well known and well documented, for example in the Bry Air design manual as well as the Munters design manual. However, with these traditional and known methods of dehumidifier capacity control, during the operation of such dehumidifier systems, reduction of the regeneration energy usage is limited. All of the above do not achieve the maximum energy reduction desirable, or to a large extent commensurate, with the changes in the instantaneous moisture load.
There are several examples of prior art practiced to reduce the regeneration energy and/or to regulate the desiccant wheel speed while optimizing dehumidifier capacity.
U.S. Pat. No. 4,546,442 teaches a microcomputer-based programmable control system for fixed bed, multi-bed desiccant air dryers commonly used to dehumidify compressed air or other compressed gases. The control system is used to monitor the level of moisture in the desiccant and determine whether a regeneration cycle is required, and also to monitor the full depressurization and repressurization of the regeneration bed, and also to analyze and indicate valve malfunction. The application of the invention is limited to a compressed air system.
U.S. Pat. No. 4,729,774 teaches the profiling of air temperature in the regeneration sector to improve dehumidifier performance.
U.S. Pat. No. 4,926,618 teaches a desiccant unit having controllable reactivation air recirculating means and variable wheel speed means. The process air humidity is controlled by a master controller modulating wheel speed, reactivation air recirculation rate and reactivation heat input. Process and reactivation airflow rates through the wheel are fixed, and the reactivation air heater is controlled to maintain a constant reactivation air temperature leaving the wheel.
U.S. Pat. No. 5,148,374 teaches a system and method for real-time computer control of multi wheel sorbent mass energy transfer systems by optimization of calculated mass transfer ratios and measures of system effectiveness which are not subject to long system time constants. The method relies on sensing at predetermined intervals a predetermined set of parameters selected from the group of wheel inlet temperature, and wheel outlet temperature, etc., to send a control signal to a predetermined one of a group of control means which includes controlling fluid flow temperature. The objective of the control method is to improve the response of the controlled device to a rapid change in load without causing unstable operation of the device and resultant fluctuations of the controlled variable.
U.S. Pat. No. 5,688,305 teaches an apparatus and method of regeneration of regeneration control for a desiccant dehumidification system in which the reactivation airflow is controlled to maintain a constant reactivation discharge air temperature and the reactivation air inlet temperature is controlled at a fixed value. The residence time of the desiccant in reactivation is also controlled in inverse proportion to the reactivation airflow. The object of this document is to reduce the over-generation of desiccant under part-load conditions, thus improving the operating efficiency of the desiccant dehumidifier. The application cited is for drying granular material in a bin or hopper using a dehumidified recirculated airstream, when the flow of granular material through the bin may occur in batches or at a variable rate.
U.S. Pat. No. 6,199,388 B1 teaches a system and method for controlling the temperature and humidity level of a controlled space and is applied mainly to a combination of an enthalpy wheel, otherwise known as energy recovery wheel, a cooling coil, and a “passive” desiccant dehumidification wheel which does not employ any external thermal heat or energy input for reactivation. It further teaches a means for changing the performance of a “passive” desiccant wheel through change in rotational speed in response to the sensible and latent loads in the controlled space. Control of the desiccant wheel speed is discussed and the intent is to control the dehumidification capacity of the “passive” wheel rather than optimize the energy efficiency of the dehumidification process. It does not teach the use of process air face and bypass dampers to control the capacity of the dehumidification wheel. Both supply (process) and exhaust (reactivation) airflows are maintained at a constant value through all loading conditions.
U.S. Pat. No. 6,355,091 B1 teaches a unitary ventilation and dehumidification system for supplying outside ventilation air to a conditioned space. The unit includes a desiccant wheel which is rotated at a slow speed to accomplish more dehumidification, and at a fast speed to accomplish more heat recovery. Heat may be added to the space exhaust air upstream of the desiccant wheel to improve its dehumidification performance and to prevent frost formation during winter operation. Both supply and exhaust airflows are fixed, no bypass dampers are used, and rotor speed adjustment is for selection of operating mode and not efficiency improvement.
U.S. Pat. No. 6,767,390 B2 teaches a method to control the performance of a multi-bed, fixed bed desiccant dryer for compressed air and compressed gas applications and to optimize the regeneration and purge cycles to deliver the gas at the desired dew point. The intended field of application is compressed air for use in instruments.
U.S. Pat. No. 7,017,356 B2 teaches about an HVAC system for cooling and dehumidifying comfort-conditioned spaces which includes a desiccant wheel in a passive dehumidification arrangement where the wheel's speed varies with airflow, and the wheel is operated for at least a set period during start up to prevent a surge of humid air into the conditioned space. This patent also teaches the use of a passive sensible recovery device and cooling coil to precondition the outside air before it mixes with the return air from the conditioned space.
U.S. Pat. No. 7,101,414B teaches a method for reducing a sorbent concentration for a process fluid stream using a sorption bed system which includes material that is rotated through multiple zones, in addition to traditional process and regeneration zones, whereby one or two pairs of independent recirculated fluid streams, other than process and regeneration flow streams, are used to isolate process and regeneration flow streams from each other. The objective of the isolation may be to prevent cross-leakage of air between process and reactivation zones, permeation of sorbate through the sorption bed, or formation of condensation or frost on the sorption bed.
U.S. Pat. No. 7,338,548 B2 teaches the use of an apparatus and a control method of conditioning humidity and temperature in a process air stream from a desiccant dehumidifier, where a portion of the process discharge air is used to preheat the regeneration air by use of an air-to-air heat exchanger. The field of use of the invention is in drying of structures and remediation of water damage.
U.S. Pat. No. 7,389,646 B2 is a divisional application for previous work and is similar to 7,017,356 B2 by the same inventor. It also is intended for cooling and dehumidifying comfort-conditioned spaces and teaches an HVAC system which includes a passive desiccant wheel, wherein the wheel's speed varies with airflow, and relies on the wheel being energized for at least a set period, at start up, and employs a heat recovery system upstream of the wheel to enhance the system's ability to dehumidify air.
Prior art control strategies have been only partially successful in limiting and reducing the use of reactivation energy. Further, during the use and application of the desiccant wheel and system, there is usually a considerable change in the instantaneous moisture loads in the fresh air and the internal latent loads within the controlled space, based on the changes of outdoor temperature and humidity, and product and occupancy loads. Therefore a need exists for a control method, along with necessary related components, that will substantially reduce the use of reactivation energy and that responds not only to changes in the dynamic/instantaneous moisture load but also simultaneously allows the optimization of energy use in the wheel, during these changes in moisture load.
To better understand the invention, first, FIGS. 1-7 are used to further provide background.
FIG. 1(a) is a typical desiccant dehumidifier flow chart, illustrating that, a typical rotating desiccant bed/wheel 1 has a process sector 2 and regeneration or reactivation sector 3. The dehumidifier incorporating such a wheel 1 would have a process flow 6, and regeneration flow 8. The regeneration flow 8 is elevated in temperature by a heat source 10 before entering the reactivation sector 3. The regeneration air, exiting the reactivation sector 3 is exhausted at 9 by regeneration blower 5. Rotation of wheel 1 is driven by a bed drive arrangement 4 that may include a drive belt 4A. In this application the term “rotational driver” or similar has the same meaning as a bed drive arrangement. There are a variety of arrangements of rotating means possible.
FIG. 1(b) illustrates a typical sector division of the wheel 1. The process sector 2, in a typical unit is 75% of the total bed area, as is shown, and can, in practice, generally vary from 50% to 80%, but can be designed to be smaller or larger. The remaining area of the wheel 1 is reactivation sector 3, and can vary between 20% and 50% but can be designed to be even smaller or larger.
FIG. 2(a) illustrates purge sector 11. The purge sector generally varies from 5 to 40% of the total bed area, the remainder being divided between process sector 2 and reactivation sector 3. When the bed rotates from the reactivation sector 3 to the process sector 2, the bed is still hot. It is well known that the hot portion of the bed, particularly if it is of the silica gel type, will begin to perform (that is, remove moisture) when it has cooled down. Therefore, a certain portion of the bed is substantially inactive in performing the dehumidifying function while it is still hot. This segment or portion of the bed is often sectioned off and made into a purge sector 11. Air 12 is made to pass over this sector 11, where the bed is hot, whereby the air 13 is preheated, before being made to pass through the reactivation sector 3, thereby both reducing the reactivation energy input needed, and also cooling that portion of the bed before entering the process sector 2, whereby the dehumidification performance through the process sector 2 is improved. In addition, less heat is imparted to the process air because the bed is cooler when it enters the process sector.
FIG. 2(b): shows the desiccant bed/wheel 1 from another angle, where various sectors are marked. Although shown in typical proportions, these areas can vary, as explained above.
FIG. 3(a): shows a flow chart of a rotary wheel 1 system embodiment where a pair of sectors (11a,12) has been added. In such a configuration it is typical to continuously circulate a given amount of airflow through these sectors, in a closed loop, with the help of a separate fan 15. The recirculated airflow acts as a buffer between the process and reactivation airstreams, capturing air leakage or moisture diffusion between the process and reactivation airstreams and thus improving the system performance. In some cases the recirculated airflow may also transfer heat between the sectors in the same manner as the purge sector shown in FIG. 2, further improving the system performance. It should be noted that the airflow in the recirculation loops described in all the figures may be in either direction, with the most advantageous direction depending on the specifics of a particular application. FIG. 3(b): shows the wheel 1 from another angle, with various sectors marked. Although shown in typical proportions, these sectors areas can vary, as clearly explained above.
FIG. 4(a) is a flow chart of a rotary desiccant bed/wheel 1 where more than one pair of purge sectors 11a, 12, 17,18 has been added. In such a configuration, it is typical to circulate a given amount of air 13, 19 through these sections, in a closed loop, by separate fans 15, 21.
FIG. 4(b) shows the wheel 1 from another angle, with various sectors marked, and also shown in a typical way, these sectors can vary, as clearly explained above.
FIG. 5(a &b) shows a typical and traditional dehumidifier system for controlling a space 27. In this system, for example, the cooling needs for the space to be dehumidified, necessitate a certain quantity of overall supply air 26 to be taken over the cooling unit or coil 24, and then supplied to the controlled space. A greater airflow may be required to satisfy the space cooling needs than needs be passed through the desiccant wheel to satisfy the space dehumidification needs. To accomplish this it is common practice to take a portion of the air through the dehumidifier and bypass 25 to make up the total supply airflow passed through the cooling coil and delivered to the space 27. There is often a need to supply fresh air 31 to meet space ventilation/pressurization requirements. The fresh air is generally introduced at the inlet to the dehumidifier, combined with the returning air 28 from the controlled space 27. It may be advantageous to cool/heat the fresh air before combining it with the return air using air heating/cooling means 22 and 23 as shown in the figure. In this typical flow chart/schematic, the fresh air flow is controlled with a damper 35. A bypass damper 32 is used to control the flow that needs to bypass the desiccant dehumidifier unit. The overall supply air flow 26 is controlled with damper 33. Each of these dampers may be adjusted manually, or automatically using actuators and appropriate controls.
Regeneration flow is controlled with a damper 34 generally positioned after the reactivation fan 5. The regeneration heat input source 10 may be electric, steam, gas or oil burner, thermal fluid such as hot water, refrigeration condenser heat, recovered heat from another process, or any combination of these that can heat the reactivation air to the temperature required for the application. The reactivation heat energy input is regulated by a thermostat 30 which is generally positioned prior (FIG. 5(a)) to the desiccant bed 1. Alternatively, thermostat 36 may be located after the desiccant bed in the reactivation “out” section as shown in FIG. 5b. In some cases the control location of FIG. 5B results in reduced annual reactivation heat use, compared with the thermostatic location of FIG. 5A.
In both the above mentioned dehumidifier systems and reactivation heat input regulation and control methods, control strategies presently used will sense the “satisfaction” of the relative humidity or moisture level of a given space, or process, or supply air, and stop the reactivation airflow, bed rotation and reactivation heat input when the humidity is satisfied, commonly referred to as “on-off” control. In another known method, commonly used with fixed temperature heat sources such as steam or hot water, the reactivation airflow is modulated to regulate the dehumidification capacity of the unit.
FIG. 6(a): shows a typical dehumidifier system used for drying applications. In this system, the dehumidified air 7 is heated by a heat source 22 as per the requirement of the material in a drying bin 37. The return air 28 carrying moisture from the product is passed over cooling coil 23 and passed through the desiccant wheel/bed 1 to adsorb the moisture.
The regeneration airflow 8 is provided by the reactivation blower 5. The heat source 10 is used to elevate the temperature based on the specific design of the unit. The reactivation inlet temperature is controlled through thermostat 30.
FIG. 6(b) shows the desiccant bed/wheel from another angle. The process sector 2, in a typical unit is 75% of the total bed area, and is shown as such, and can, in practice, generally vary from 50% to 80% but can be designed to be even smaller or larger. The remaining area of desiccant bed is shown as the reactivation sector 3, and can vary between 20% and 50% but can be designed to be even smaller or larger.
FIG. 7(a) Shows a second typical dehumidifier system for a drying application.
This is similar to the system explained in FIG. 6 (a&b), except a purge sector 11 has been added. This purge sector can vary from 5 to 40% of the total bed area. The object of using a purge sector has already been explained previously.
FIG. 7(b) shows desiccant bed/wheel 1 from another angle, where the various sectors are marked, and although shown in a typical way these sectors areas can vary, as explained above.